Electrolytical process of preparing silver halides from metallic silver



Oct. 12 1926. 1,602,595

s. E. sHEPPARD ET A1.

ELECTROLYTLCAL PROCESS OF P REPARlNG SILVER HALIDES 'FROM METALLIC SILVER Filed Feb. 27, 1926 I femm/ed BY Cil Patented Oct. 12, 1926.

UNITED STATES PATENT OFFICE.

SAMUEL E. SHEPPABD AND RAYHOND'H. LADEBT, OF ROCHESTER, NEW YORK, AS- SIGNOBS T EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A CORPORA- TION OF NEW YORK.

ELECTROLYTICAL PROCESS OF PBEPABING SILVER HALE-)ES FBOI IETALLIC Application lcd February 27, 1926. Serial No. 91,215.

This invention relates to electrolytic processes of preparing silver halides from metallic silver. One object of the invention is to provide a simplified process which will avoid expensive operations now used in preparingT such halides. Another object is to provide a process in which the formation of intermediate silver compounds is unnecessary. A still further object is to provide a process for preparing such halides in a finegrained. deflocculated form readily purified by .washing and suitable for photographic or pharmaceutical purposes. Another object of the invention is to provide an electroytic process for preparing halides in which the electrolyte passes throught a cycle which avoids waste of silver values. `Yct another' object of the invention is to provide au electrolytic process in which alkalization of the anolyte is usefully opposed. Other objects will hereinafter appear.

ln the accompanying drawing.-

Fig. 1 is a diagrammatic vertical sectional view. upon an exaggerated scale. of an apparatus in which our process may be carried out;

Fig. E! is a diagrammatic vertical sectional view of another-'form of apparatus in which our process may be carried out:

Figin is a mill diagram illustrating one form of our process.A 4

Silver halides for photographic and pharmaceutical use are usually prepared by precipitation when a solution of a soluble silver salt has been mixed with a solution of a soluble halide. The soluble silver Salt which is most commonly employed is the nitrate. The preparation of this salt in a pure form requires thatl the silver be dissolved in nitric avid of stringent chemical urity. From this solution vsilver nitrate is o tained by tedious crystallization andrecrystallization. (lur process avoids this production of an intermediate salttogether with its tedious and expensive operations.

After the puresilver nitrate is dissolved and silver halides are precipitated. from this solution. such halides lmust be very thoroughly washed. We have found that the silver halides produc-ed byour process may be washed free from the associated products of the reaction moreeasily than in the case-'of the silver halides produced in the conventional manner.

.Our process. is concerned with silver bromide, silver iodide and silver chloride and not with silver fluoride. Since bromide. iodine and chlorine are all of hi her atomic welght than uorine, we shall, or convenience. hereinafter designate them as a grou as well as their silver and other com oun s,`

by the adjective heavy,-'thus a` heavy halide of silver means either silver bromide, iodide, or chloride and not the fluoride.

We have discovered that the hereinabove recited objects may be attained in the preparation of the heavy halides of silver by converting anode silver into such halides in aqueous electrolytes which are solvents of to normal so that it can be reused in treating In this way the proce further anode silver. ess may be made continuous by passing the electrolyte through this' circle` and waste of heavy halide anions is practically eliminated, as well as waste of unprecipitated silver halide.

The electrolytes which we refer are solvents of the heavy silver halides by reason of their concentrated content of one or more of the heavy halides of the alkali metals, or by reason of the high temperature of the electrolyte, or preferably bot-h. We shall hereinafter include in the term alkali metal ammonium the salts of which function. in our process. in the same way that `the salts of the alkali metals do. The heavy halide alkali salt or salts which are resent in the electrolyte. correspond to the eavy silver halide or halides to be produced. Thus when makin silver bromide we employ a hot concentrat bath of a bromide of an alkali metal. It is preferable to use saturated solutions of the alkali salts and to maintain them during the electrolysis at a temperature between C. and boiling. We have found C. to be convenient, but temperatures up to the boiling point of thel solution may be used to give heavier yie ds.

While 'the principle of the process is applicable to the production of silver chloride, as well as silver bromide and silver iodide, the process is much more easily carried out in the case of the two latter compounds, because it is easier to prepare suitable electrolytes for their production which have the necessary solvent power.

Since silver bromide is of the widest technical importance, we shall give examples of our process in the manufacture of this oompound, but it will be understood that our invention is not restricted to the details thus given, except as indicated in the appended claims.

Referring to Figure 1, in the vessel l is located the electrolyte 2. This may be, for example, a saturated aqueous solution of potassium bromide heated to 80 C. in any suitable manner, sayby burner 3. Suspended in the electrolyte are the metallic silver anode 4 and cathode 5. The cathode may be of any conductor which will not rontaminate the bath, but a silver-cathode gives good results and high current `eiliciency The current ma be direct or even a pulsating current o tained by'superposing an alternating current upon a direct current.

The current density maybe varied over av considerable range. It is not essential tlat 1t be kept uniform throughout the electrolysis, so long as it is maintained within practical limits. An initial current density of 6.5 amperes per square decimeter illustrates a good average value. In the arrangement shown in Figure 1, this current density tends to rise during the electrolysis, where constant current is maintained by suitable regulators, because the area of the anode is progressively decreased as it dissolves away. As is well known in the art, a constant current density can be maintained by feeding bar anodes into the electrolyte either by hand or automatically. As such devices are well known, nodescription of them is needed.

The electrolysis is carried on until the electrolyte is practically saturated with silver bromide, which is dissolved by the bath as fast as it is formed at the anode. The end point can be indicated by testing` the electrolyte at intervals, or by .noting the appearance of the anode when the silver bromide is not dissolved with suilicient rapidity. lVhen the content of silver bromide in the electrolyte has reached a practical amount the electrolyte is drawn out, say through valve 6,- and both cooled and diluted with water, preferably at or below room temperature.

naoaeae This causes a ne-grained flocculent precipitate of bromide which may be readily separated from the electrolyte, by settling and deeantation, or by filtering, or by any other of the 'known processes of separating liquids and their precipitates Filtering is convenient and .the filtered silver bromide is easily washedsubstantially free from potassium bromide and any other salts present or formed in the electrolyte during the process. We have found that a precipitate, obtained in the manner indicated above, is purified especially readily with the minimum amount of washing. Plain cold Water is effective, although after the bulk of the potassium bromide is washed away, warm water may be employed in the purification, if desired. Minimum Washing lessens loss of silver values.

The diluted electrolyte is then restored to substantially its initial condition by reheating it -to 80 C. and resaturating it with potassium bromide. This makes it ready for the further treatment of anode silver. The bath thus functions through a regular cycle. This brings about a very important saving, because any silver bromide which is not precipitatedout from it is ren Stored to the electrolysis vessel, and thus there is no leak of silver values. Potassium bromide may be added, or water driven of, or both, when resaturatin the bath.

v During each electrolytic operation. the catholyte around the cathode 5 becomes basic in charaeter."'The hydroxyl ions thus pro- 10J duced, may, in time, migrate into the anolyte and interfere with the production of silver bromide and its dissolution in the bath. We have found that this can be corrected bystirring in small amounts of hydrobromic in.; acid, as needed, to keep the anolyte in proper condition free from excessive alkalinity. We have indicated a source of liydrobromic acid diagrammatically at 7 in Figure 1. ln this way practically current etliciency can 11o be maintained.

ll'e can not only oppose the alkalization ofthe anolyte by neutralizing with the proper heavy halide acid, but can likewise oppose it by using a suitable porous diaphragm. The layout of such a system is shown diagrammatically in Figure 2. ln this view the vessel 8 contains the electrolyte 9, which may be a saturated solution of potassium bromide, as in the apparatus of Figure '1. The anolyte and catholyte may be initially the same. The electrolyte may be maintained at suitable temperature, say 80 C. by heating jacket l0, 1n .which steam, hot water, heated oil or other heat carrier may be employed. The porous diaphragm is indicated at 11 conventionally supported at 12. It may be cylindrical with the silver anodes 13 arranged around it. lVhile any kind of porous diaphragm, not

of the attacked by hot solutions of the salts used, are suitable, we prefer unglazedhporcelain, alundum, sintere glass or the e. lny

suitable cathode 14 may be employed. When the content of silver bromide has increased to a desirable proportion in the electrolyte, the latter may be drawn of throu'ghvalve l5. In case a'nundue portion of hydroxyl ions migrate through the diaphragm 11 into the anolyte surrounding anodes 13, said anolyte may be acidified by using hydrobrothe anodes .may be fed into the bath automatically in any suitable way. We prefer to move or agitate the electrolyte adjacent` the anode where the silver halide is formed and dissolved. In the drawing we have conventionally 'represented electric motors 17 geared to stirrers 18.

lVhen preparing silver bromide by our process, we have found that over 6% of the silver vbromide. may be` dissolved in the saturated silver bromide at 80 C. and over 10% at 100 C., which is suilicient to be technically important and usable. This may be slightly below saturation at the temperature employed but is, nevertheless, Ipractical. Then the electrolyte is both cooled and diluted the loss in solvent-power toward silver bromide is -very marked and a good extraction by precipitation is, therefore, possible. For dilution we use water free from troublesome impurities, say .distilled water.

The process, especially when preparing heavy halides of silver for photographic use,

is preferably carried out 1n non-actinic or .safe light, of a suitable yellow or red color.

rl`he electrolysis may be carried out in ordinary daylight or the usual artificial lights, the recipitation and all subs uent steps shou d be done in a safely lighte room. b

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is: 4

1. The process of re aring a heavy halide of silver which inc u es the steps of elect'rolytically` converting anode silver into silver halide inan aqueouselectrolyte which is a solvent of said sllver halide and contains heavy halide anions, reducing the solvent power of said electrolyte. to precipitate silver halide dissolved during said first-named step, and separating the precipitate from said electrolyte.

2. The cyclic process of reparing a heavy halide of silver which inc udes the steps of electrelytically Aconverting anode silver into silver'halide in an aqueous electrol. e which heavy halide lanions, reducing the solvent power of said electrolyte to precipitate silver alide dissolved during said first-named step, separating the precipitate from said electrolyte, and restoring the solvent power and halide anion content ofvsaid electrolyte for treating further anode silver.

3. The process of preparing a heavy halide vof silver which includes the steps of electrolytically converting anode silver into silver halide in an aqueous electrolyte containingy heavy halide anions heated above room temperature, said electrolyte having greater solvent power above room temperature than at or below room temperature for said silver halide, lowering the temperature of vsaid electrolyte until silver halide ,dissolved during said rst step is precipitated, separating the recipitate from said electrolyte, and puri y ing it by washing.

4. The 'process of reparing aheavy halide of silver which inc udes thesteps of electrolytically converting anode silver'into silver halide in an aqueous electrolyte containing heavy halide anions4 heated above room temperature, said electrolyte having greater tially saturated with the correspondingl heavy halide of an alkali metal above room temperature, cooling and diluting said electrolyte to precipitate silver halide dissolved durlng said rst-named step, and separating the precipitate from said electrolyte.

6. The process of preparing a heavy halide of silver which includes the steps of electrolytically converting anode silver into silt ver halide in an aqueous electrolyte containing heavy halide anions heated above room temperature, said electrolyte having greater solvent power above room temperature than at or be ow room temperature `for said silver halide, lowering theV temperature of said electrolyte until silver halide dissolved during said first step is precipitated, separating the precipitate from said electrolyte, and purifying it by washing, the halide anion content and the temperature of said electrolyte thereafter restored for the treatment of v being furt 1er `anode silver, said electrolyte beine' thereafter resaturated with said alkali halide' and-restored in temperature for the treatis a solvent of saidV silver halide an contains ment of further anode silver. i K

7. The process of preparing silver bromide which includes the steps of electrolytically converting anode silver into silver bromide in an aqueous electrolyte which is a solvent of silver bromide and contains bromide ions, reducing the solvent power of said electrolyte to precipitate silver bromide dissolved during said first-named step, and separating the precipitate from said electrolyte.

8. The processv of preparing silver bromide which includes the steps of electrolytically converting anode silver into silver bromide in an aqueous electrolyte which is a solvent of silver bromide and contains bromide ions,-reducing the solvent povver of said electrolyte to precipitate silver bromide dissolved during said first-named step.' and `separating the precipitate troni-said electrolyte, the solvent power and bromide ion content of said electrolyte being restored lfor treatment of further anode silver.

- ous electrolyte of alkali 9. The process of preparing silver bromide which comprises the step of electrolytically attacking a silver anode in an aqueous electrolyte having suiciently high bromide ion concentration to dissolve the resulting silver bromide as it is formed.

10. The process of preparing silver bro' mide which comprises' the step of electro-w lytically attacking anode silver in an aquebromide heated above room temperature which dissolves the resulting silver bromide as it is formed.

11. The process of preparing k,a heavy halide of silver which includes t-he step of electrolytically dissolving a silver anode in an electrolyte comprising a saturated aqueous solution of a heavy halide of an alkali metal at a-temperature from 70 C. to boiling.

12. The process of preparing silver bromide which includes electrolytically dissolving a silver anode in an electrolyte comprising an aqueous saturated solution of an alkali bromide at a temperature from 70 C. to boiling, and precipitating, separating' and Washing the resulting silver bromide from` said solution.

13. The cyclic process of preparing silver bromide which includes electrolytically dissolving a silver anode in an electrolyte comprising initially an aqueous saturated solution of an alkali bromide at a temperature C. to boiling, precipitating and separating the resulting silver bromide from said solution, and bringing said electrolyte to its initial condition for treating further silver anodes.

14. The process of preparin a. heavy halide of silver which includes t e steps of electrolytically converting anode silver into said silver halide in an aqueous electrolyte containing sufficient corresponding heavy halide oan alkali metal to dissolve said silver halide, and opposing the anolyte by the catholyte.

15. The process of preparing a heavy halide of silver which includes the steps of electrolytically converting anode silver into said silver halide in an aqueous electrolyte containing suiicient corresponding heavy halide of an alkali metal to dissolve said -silver halide, and adding the corresponding .heavy halide acid to the electrol 'te.

l 16. The process of preparing-*a heavy halide of silver which includes the steps of electrolytically converting anode silver into said sliver halide in an anolyte containing sufiicient corresponding heaiqy halide of an alkali metal to dissolve said silver halide, said anolyte being-separated by a porous diaphragm from the catholyte.

17. The process of preparing silver broalkalization of mide which includes the steps of electro- I vtically attacking anode silver in an aqueous electrolyte containing sucient alkali bromide to dissolve the silver bromide, and opposing alkalization of the anolytev by the catholyte.

18. The process of preparing silver'bromide which includes the steps of electrolytically attacking anode silver in an anolyte of aqueous alkali bromide solution said anolyte being suciently concentrated and heated to dissolve the silver bromide thus formed, and adding hydrobromic acid to said anolyte. Y

19. The process of preparing silver bromide which includes theV steps of electrolytically attacking anode silver in an electrolyte o/f aqueous alkali bromide solution, the anolyte being separated from the catholyte by a porous diaphragm and vsaid anolyte bein suciently concentrated and heated to disso ve the silver bromide thus formed.

20. The process of preparing silver bromidewhich includes lthe/steps of electrolytically attacking anode silver in an electro` lyte of saturated aqueous alkali bromide solution, the anolyte being separated from the catholyte by a porous diaphragm and said anolyte being heated between C. and boiling.

21. The process of preparing silver bromide which includes the steps of electrolytically attacking anode silver in an electrolyteof saturated aqueous alkali bromide solution, the anolyte being separated from the catholyte by a porous diaphragm and said anolyte being heated betvveen 70 C. and boiling, and adding hydrobromic acid to said analyte to counteract alkali migrating through the diaphragm from the catho- Signed at Rochester, New York, this 22nd day of February, 19:26.

SAMUEL E. SHEPPARD. RYMND H. Lili' lBER'l. 

